In recent years, as a battery anticipated to have a small size, a light weight, and high capacity, a non-aqueous electrolytic solution-based secondary battery such as a lithium ion battery has been proposed and put into practical use. The lithium ion battery is configured to have a positive electrode and a negative electrode which have properties capable of reversibly intercalating and deintercalating lithium ions, and a non-aqueous electrolyte.
As a negative electrode active material for negative electrode materials of the lithium ion battery, a lithium-containing metal oxide such as lithium titanate (Li4Ti5O12) or a carbon-based material is used.
On the other hand, as a positive electrode active material for positive electrode materials of the lithium ion battery, a lithium-containing metal oxide such as lithium cobaltate (LiCoO2), a lithium oxoacid salt-based compound such as lithium iron phosphate (LiFePO4), or the like is used. The positive electrode of the lithium ion battery can be formed by, for example, applying and drying paste for electrodes including a positive electrode active material, a binding agent, and the like on the surface of a metal foil called a collector.
Compared with secondary batteries of the related art such as lead batteries, nickel-cadmium batteries, and nickel-hydrogen batteries, the lithium ion batteries have a lighter weight, a smaller size, and higher energy. Therefore, the lithium ion batteries are used not only as small-size power supplies used in portable electronic devices such as mobile phones and notebook personal computers but also as large-size stationary emergency power supplies.
In addition, recently, studies have been underway regarding the use of lithium ion batteries as high-output power supplies for plug-in hybrid vehicles, hybrid vehicles, industrial device, electric power tools, and the like. Since batteries used as the above-described high-output power supplies are frequently used in applications that are expected to be used outdoors, the batteries are required to have high-speed charge and discharge characteristics not only at room temperature but also at a low temperature in a case in which the batteries are expected to be used in cold areas.
Among the electrode active materials, the lithium oxoacid salt-based compound (particularly, lithium iron phosphate) is attracting attention due to the excellent safety and the absence of problems with its resource and cost. However, the lithium oxoacid salt-based compound has a problem with low electron conductivity due to its crystal structure (olivine-type crystal structure).
Therefore, in order to increase the electron conductivity of an electrode material for which the lithium oxoacid salt-based compound is used as an electrode active material, means of Japanese Laid-open Patent Publication No. 2001-15111 has been proposed. In Japanese Laid-open Patent Publication No. 2001-15111, particle surfaces of an electrode active material made of lithium iron phosphate are covered with an organic compound that is a carbon source, then, the organic compound is carbonized so as to form a carbonaceous film on the surface of the electrode active material, and carbon in the carbonaceous film is made to act as an electron conductive substance.